1. Field of the Invention
The present invention pertains to electrochemical devices such as solid oxide fuel cells (SOFCs) or ceramic fuel cells, particularly thin film solid oxide fuel cells (TFSOFCs). More particularly, a porous metallic anode and a thin film conducting oxide porous cathode are provided, along with methods for forming the electrodes and a thin film electrolyte.
2. Description of Related Art
Fuel cells are energy-converting devices that use an oxidizer (e.g. oxygen in air) to convert the chemical energy in fuel (e.g. hydrogen) into electricity. A SOFC (also called a “ceramic fuel cell”) generally comprises a solid electrolyte layer with an oxidizer electrode (cathode) on one side of the electrolyte and a fuel electrode (anode) on the other side. The electrodes are required to be porous, or at least permeable to oxidizer at the cathode and fuel at the anode, while the electrolyte layer is required to be dense so as to prevent leakage of gas across the layer. A TFSOFC has a thin electrolyte layer, on the order of 1-10 micrometers thick, as described, for example, in U.S. Pat. No. 5,753,385. This reduces the ohmic resistance of the electrolyte and increases the power density of the fuel cell. Because of the low electrolyte resistance, the TFSOFC can operate at lower temperatures. This increases the reliability and allows wider choices of materials for TFSOFC applications. Using the TFSOFC design can also reduce materials costs and reduce the volume and mass of the fuel cell for a given power output.
U.S. Pat. No. 5,753,385 discloses physical and chemical deposition techniques to synthesize the basic components of a TFSOFC. In one technique, the electrodes are formed from ceramic powders sputter coated with an appropriate metal and sintered to a porous compact. The electrolyte is formed by reactive magnetron deposition. The electrolyte-electrode interface is formed by chemical vapor deposition of zirconia compounds onto the porous electrodes.
U.S. Pat. No. 5,656,387 discloses an improved nickel and yttrium-stabilized zirconia (YSZ) anode and a method for making by DC magnetron sputtering. The films were deposited on a surface of yttria-stabilized zirconia (YSZ).
U.S. Pat. No. 5,106,654 discloses a method for matching thermal coefficients of expansion in fuel cell or other electrochemical devices. A tubular configuration not employing thin films is described.
YSZ thin film fuel cells have generally been formed by depositing the YSZ on a substrate that is not crystallographically ordered. Therefore, the YSZ is not ordered and thicker layers must be deposited to form a layer impermeable to gas. Michibata et al (“Preparation of Stabilized Zirconia Electrolyte Films by Vacuum Evaporation,” Denki Kagaku, 58, No. 11 (1990) demonstrated growth of dense but not atomically ordered YSZ films on nickel foil. They also provided no mechanism for increasing gas permeability of the nickel foil and claimed very low maximum power output (7 mW/cm2) of a resulting fuel cell.
To make thin film solid oxide fuel cells more efficient and less expensive to fabricate, improved methods for forming the porous electrodes and the non-porous electrolyte used in such devices are needed. The electrolyte should be defect-free to avoid charge and gas leakage across the cell, and thin to provide lower electrical resistance at moderate temperatures. Interconnect layers to make possible stacking of cells should be provided.